1. Field of the Invention
The invention relates to integrated circuits and more specifically to a starting circuit or power-on-reset circuit that can be used for the temporary inhibition of certain functions of the integrated circuit when the power is being turned on. Indeed, in certain logic circuits, erroneous operations may be caused by the insufficiency of the power supply. To prevent these erroneous operations, it is preferred to totally inhibit certain functions of the integrated circuit while the power is being turned on.
A power-on-reset circuit is used for this purpose. This circuit produces a signal that starts when the power is turned on and continues until the supply voltage has reached a sufficient value. This square-wave signal is applied to an input for the inhibition of each of the circuits whose operation has to be inhibited temporarily.
Typically, for a nominal supply voltage of five volts, steps are taken to produce an inhibition signal that starts when the power is turned on and generally continues for a period ranging from some milliseconds to some tens of milliseconds after the supply voltage has gone beyond a threshold of about 3 to 3.3 volts which is acceptable for the working of the integrated circuit.
A power-on-reset circuit appears, for example, to be indispensable for non-volatile, electrically erasable and programmable memory (EEPROM) circuits, especially when such a memory is organized into words of several bits that are written or erased in parallel, since there is then a risk of undesirable programming of the memory as a result of unreliable instructions given at the start of the powering of the circuit. This is of critical importance, for example with EEPROM chip card circuits where the contents of the memory have to be perfectly reliable.
2. Description of the Prior Art
In general, in power-on-reset circuits of the prior art, the output voltage of the power-on-reset circuit, namely the voltage applied to an inhibition input of a circuit whose operation has to be prohibited, takes the following course: in the beginning, it follows the supply voltage fairly closely and then, when the supply voltage exceeds a determined threshold, a time delay is triggered so that the inhibition signal lasts for a certain period of time, after which it gets interrupted. If the supply voltage then varies and momentarily falls below the threshold, the inhibition signal may reappear and it then follows the variations of the supply voltage, but without re-triggering a time delay.
It has been observed that, in certain cases, the reliability of the integrated circuit remains insufficient.